The objective is to document the effects of aging on hymenal morphology during the first three years of life in a cohort of nonabused girls. Using a longitudinal design, we examined and photographed the external genitalia of 134 girls at 2 months or less and near 3 years of age; 42 of these girls were also examined near 1 year of age. The prevalence of each hymenal characteristic was calculated at each time, and differences were analyzed using the z statistic and McNemar change tests. Measurements of transhymenal diameters and the inferior rims were compared using a paired t-test. Hymenal configuration in 65% (87 of 134) of the subjects changed between birth and 3 years, usually from annular or fimbriated to cresentic. External ridges observed at birth usually resolved by three years, whereas intravaginal ridges were observed more often in the 3-year olds (P=.00). Analysis by race showed that the prevalence of both superior and lateral notches decreased in whites, whereas prevalence of intravaginal ridges changed only in blacks (P=.00). Sixty-eight percent (15 of 22) of the tags present at birth were not observed at 3 years, while nine tags formed during this period. Changes observed between 1 and 3 years included increases in the mean horizontal (P=.00) and vertical (P=.02) transhymenal diameters and in the prevalence of the cresentic configuration (P=.04). The significance is that changes in hymenal morphology, which may vary by race, occur in the first three years of life. Alterations are more pronounced in the first year than in years 2 and 3. Physicians should understand the effects of aging on the hymen's appearance to differentiate normal development from post-traumatic or infectious changes. This study is designed to follow a single cohort of female children until age 12. The girls will visit the General Clinical Research Center again in the spring and summer of 1996 when the girls will be approximately 7 years old.